bobview 1.9.0

//! The strip chart widget: its PVs plotted against time, live. Each PV
//! keeps the samples it has had since the display opened; there is no
//! archived history. (Trend windows are archiveview plots: see `trend`.)

use std::collections::VecDeque;
use std::sync::Arc;

use parking_lot::Mutex;
use web_time::{SystemTime, UNIX_EPOCH};

use chrono::{Local, TimeZone};
use eframe::egui::{self, Align2, Color32, FontId, Pos2, Rect, Stroke};
use pv::{Listener, Pv, PvState};

use crate::axis::{self, Scale};
use crate::plot::{self, text_size, vertical_text};
use crate::screen::Node;
use crate::style::{self, color, named};
use crate::widgets::{alarm_border, fill};

/// Samples kept per PV: at most this many (1.6 MB), and none older than a
/// day.
const MAX_SAMPLES: usize = 100_000;
const MAX_AGE: f64 = 86_400.0;

/// One PV's samples, in time order: (seconds since 1970, value), a NaN
/// value marking where it disconnected.
#[derive(Default)]
pub struct Samples {
    points: VecDeque<(f64, f64)>,
    /// How many have ever been added, so a [`Thinned`] knows which are new.
    pushed: u64,
}

impl Samples {
    pub fn push(&mut self, t: f64, v: f64) {
        // One gap is enough, and a trend starts with a value.
        if v.is_nan() && self.points.back().is_none_or(|p| p.1.is_nan()) {
            return;
        }
        // The clock may step back; the samples stay in order.
        let t = self.points.back().map_or(t, |p| t.max(p.0));
        self.points.push_back((t, v));
        self.pushed += 1;
        while self.points.len() > MAX_SAMPLES
            || self.points.front().is_some_and(|p| p.0 < t - MAX_AGE)
        {
            self.points.pop_front();
        }
    }

    /// The samples from `from` on, and the one before it, which holds at
    /// `from`.
    pub fn since(&self, from: f64) -> impl Iterator<Item = (f64, f64)> + '_ {
        let i = self.points.partition_point(|p| p.0 < from);
        self.points.range(i.saturating_sub(1)..).copied()
    }

    #[cfg(test)]
    fn len(&self) -> usize {
        self.points.len()
    }
}

/// One PV's samples thinned for drawing: in each column of time `dt` wide
/// (a pixel or less), the first, lowest, highest and last sample, which
/// draw the same as all of them. Kept from frame to frame, taking in only
/// the samples added since, so a day of fast samples is not copied out
/// and stepped through on every frame.
#[derive(Default)]
struct Thinned {
    dt: f64,
    /// The start of the time it was built for.
    from: f64,
    /// [`Samples::pushed`] when it last took samples in.
    seen: u64,
    /// The finished columns' points, in time order, NaN marking gaps.
    done: VecDeque<(f64, f64)>,
    /// The column being filled.
    open: Option<Column>,
}

/// A column's first, lowest, highest and last sample.
#[derive(Clone, Copy)]
struct Column {
    index: i64,
    first: (f64, f64),
    low: (f64, f64),
    high: (f64, f64),
    last: (f64, f64),
}

impl Column {
    /// Its points in time order, each once.
    fn points(&self) -> impl Iterator<Item = (f64, f64)> {
        let mut pts = [self.first, self.low, self.high, self.last];
        pts.sort_by(|a, b| a.0.total_cmp(&b.0));
        let mut n = 0;
        for i in 0..pts.len() {
            if n == 0 || pts[i] != pts[n - 1] {
                pts[n] = pts[i];
                n += 1;
            }
        }
        pts.into_iter().take(n)
    }
}

impl Thinned {
    fn new(dt: f64, from: f64) -> Self {
        Self {
            dt,
            from,
            ..Default::default()
        }
    }

    fn close(&mut self) {
        if let Some(c) = self.open.take() {
            self.done.extend(c.points());
        }
    }

    fn add(&mut self, (t, v): (f64, f64)) {
        if v.is_nan() {
            // A gap ends the column's line.
            self.close();
            self.done.push_back((t, v));
            return;
        }
        let index = (t / self.dt).floor() as i64;
        match &mut self.open {
            Some(c) if c.index == index => {
                if v < c.low.1 {
                    c.low = (t, v);
                }
                if v > c.high.1 {
                    c.high = (t, v);
                }
                c.last = (t, v);
            }
            _ => {
                self.close();
                self.open = Some(Column {
                    index,
                    first: (t, v),
                    low: (t, v),
                    high: (t, v),
                    last: (t, v),
                });
            }
        }
    }

    /// Bring it up to date with `samples` for the time from `from` in
    /// columns `dt` wide, rebuilding it only if the columns changed.
    fn update(&mut self, samples: &Samples, from: f64, dt: f64) {
        let new = samples.pushed - self.seen;
        if self.dt != dt || from < self.from || new > samples.points.len() as u64 {
            *self = Self::new(dt, from);
            for p in samples.since(from) {
                self.add(p);
            }
        } else {
            let len = samples.points.len();
            for p in samples.points.range(len - new as usize..) {
                self.add(*p);
            }
        }
        self.seen = samples.pushed;
        // Forget what has scrolled off, but the point that holds at `from`.
        let i = self.done.partition_point(|p| p.0 < from);
        if i > 1 {
            self.done.drain(..i - 1);
        }
    }

    fn points(&self) -> impl Iterator<Item = (f64, f64)> + '_ {
        self.done
            .iter()
            .copied()
            .chain(self.open.iter().flat_map(Column::points))
    }
}

/// The quickest a plot redraws for time moving on alone: a display redraws
/// all of itself each frame, so a strip chart must not keep it at the
/// 10 frames a second PV updates are held to.
const MIN_SCROLL_REPAINT: f64 = 0.25;

/// How long until a plot of `span` seconds over `width` points needs a
/// redraw for time moving on: when it has scrolled a pixel, but not more
/// often than [`MIN_SCROLL_REPAINT`] nor less than once a second.
fn scroll_repaint(span: f64, width: f32) -> std::time::Duration {
    let per_pixel = span / width.max(1.0) as f64;
    std::time::Duration::from_secs_f64(per_pixel.clamp(MIN_SCROLL_REPAINT, 1.0))
}

/// The time now, in seconds since 1970.
pub fn now() -> f64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map_or(0.0, |d| d.as_secs_f64())
}

/// What a PV update adds to its trend: its value, a gap if it disconnected,
/// or nothing if it has no number (yet).
fn sample(s: &PvState) -> Option<f64> {
    if s.connected {
        s.value.as_f64()
    } else {
        Some(f64::NAN)
    }
}

/// A PV being trended: a sample on each update, at the time it arrived.
pub struct Series {
    pub pv: Pv,
    /// Locked by the listener while the PV's state is locked, so never read
    /// `pv.state()` while holding this lock.
    samples: Arc<Mutex<Samples>>,
    /// The samples as last drawn.
    thinned: Mutex<Thinned>,
    _listener: Listener,
}

impl Series {
    pub fn new(pv: Pv) -> Self {
        let samples = Arc::new(Mutex::new(Samples::default()));
        let into = samples.clone();
        let listener: Listener = Arc::new(move |s: &PvState| {
            if let Some(v) = sample(s) {
                into.lock().push(now(), v);
            }
        });
        pv.listen(&listener);
        Self {
            pv,
            samples,
            thinned: Mutex::default(),
            _listener: listener,
        }
    }

    /// The points to draw from `from` to `to`, `dt` seconds a pixel: the
    /// samples thinned to a few per pixel, with the last value held to `to`
    /// while the PV is connected.
    fn points(&self, from: f64, to: f64, dt: f64) -> Vec<(f64, f64)> {
        // Copied out before the PV's state is read: an update holds that
        // lock while it adds a sample.
        let mut pts: Vec<(f64, f64)> = {
            let mut thinned = self.thinned.lock();
            thinned.update(&self.samples.lock(), from, dt);
            thinned.points().collect()
        };
        if let Some(&(t, v)) = pts.last()
            && v.is_finite()
            && t < to
            && self.pv.state().connected
        {
            pts.push((to, v));
        }
        pts
    }
}

/// A line to draw, and how.
pub struct Line<'a> {
    pub series: &'a Series,
    pub color: Color32,
    pub width: f32,
}

/// How the plot looks.
pub struct Look {
    pub log: bool,
    /// A fixed y range, or `None` to fit the data.
    pub range: Option<(f64, f64)>,
    pub title: String,
    pub grid: bool,
    pub fg: Color32,
    pub back: Color32,
    pub font: FontId,
    pub title_font: FontId,
}

/// Plot `lines` over the `span` seconds up to `now`, with a time axis below
/// and a value axis on the left.
pub fn draw_plot(
    painter: &egui::Painter,
    rect: Rect,
    lines: &[Line],
    span: f64,
    now: f64,
    look: &Look,
    z: f32,
) {
    let painter = &painter.with_clip_rect(painter.clip_rect().intersect(rect));
    fill(painter, rect, look.back);
    let (pad, tick) = (4.0 * z, 5.0 * z);
    let from = now - span;
    // Columns no wider than a pixel of the plot, which is narrower than `rect`.
    let dt = span / rect.width().max(1.0) as f64;
    let data: Vec<Vec<(f64, f64)>> = lines
        .iter()
        .map(|l| l.series.points(from, now, dt))
        .collect();

    let ok = |v: f64| v.is_finite() && (!look.log || v > 0.0);
    let in_view = data
        .iter()
        .flat_map(|pts| pts.iter().filter(|p| p.0 >= from).map(|p| p.1));
    let (lo, hi) = look
        .range
        .or_else(|| plot::autoscale(look.log, in_view, true))
        .unwrap_or(if look.log { (1.0, 10.0) } else { (0.0, 1.0) });

    let label_h = text_size(painter, "0", &look.font).y;
    let bottom = rect.bottom() - pad - label_h - tick;
    let top = rect.top() + pad + label_h / 2.0;
    let ys = Scale {
        lo,
        hi,
        log: look.log,
        from: bottom,
        to: top,
    };
    let (y_ticks, y_step) = ys.ticks(label_h * 2.5);
    let y_labels: Vec<(f64, String)> = y_ticks
        .iter()
        .map(|v| (*v, axis::label(*v, y_step)))
        .collect();
    let label_w = y_labels
        .iter()
        .map(|(_, l)| text_size(painter, l, &look.font).x)
        .fold(0.0, f32::max);
    let title_w = if look.title.is_empty() {
        0.0
    } else {
        text_size(painter, &look.title, &look.title_font).y + pad
    };
    let time_w = text_size(painter, "00:00:00", &look.font).x;
    let left = rect.left() + pad + title_w + label_w + pad / 2.0 + tick;
    let right = rect.right() - pad - time_w / 2.0;
    let plot = Rect::from_min_max(Pos2::new(left, top), Pos2::new(right, bottom));
    if plot.width() < 4.0 || plot.height() < 4.0 {
        return;
    }
    let xs = Scale {
        lo: from,
        hi: now,
        log: false,
        from: plot.left(),
        to: plot.right(),
    };
    let max_ticks = (plot.width() / (time_w + 20.0 * z)).floor().max(2.0) as usize;
    let (t_ticks, t_step) = time_ticks(from, now, max_ticks, local_offset(now));

    if look.grid {
        let grid = Stroke::new(1.0, named("Grid").gamma_multiply(0.4));
        for t in &t_ticks {
            let x = xs.pos(*t);
            painter.line_segment(
                [Pos2::new(x, plot.top()), Pos2::new(x, plot.bottom())],
                grid,
            );
        }
        for (v, _) in &y_labels {
            let y = ys.pos(*v);
            painter.line_segment(
                [Pos2::new(plot.left(), y), Pos2::new(plot.right(), y)],
                grid,
            );
        }
    }

    let clipped = painter.with_clip_rect(painter.clip_rect().intersect(plot));
    // Far-off points are pulled in so the maths stays finite.
    let bounds = plot.expand(plot.width().max(plot.height()) * 4.0);
    for (line, pts) in lines.iter().zip(&data) {
        let screen = |(t, v): (f64, f64)| {
            ok(v).then(|| {
                Pos2::new(
                    xs.pos(t).clamp(bounds.left(), bounds.right()),
                    ys.pos(v).clamp(bounds.top(), bounds.bottom()),
                )
            })
        };
        let pts: Vec<Option<Pos2>> = pts.iter().map(|p| screen(*p)).collect();
        // A value holds until the next sample, so the line steps; a gap
        // (disconnected, or not a number) breaks it.
        for run in pts.split(Option::is_none).filter(|r| !r.is_empty()) {
            let mut steps: Vec<Pos2> = Vec::with_capacity(run.len() * 2);
            for p in run.iter().flatten() {
                if let Some(last) = steps.last().copied() {
                    steps.push(Pos2::new(p.x, last.y));
                }
                steps.push(*p);
            }
            plot::line(
                &clipped,
                plot::decimate(steps),
                Stroke::new(line.width, line.color),
                0,
            );
        }
    }

    // The axes over the plot's edges.
    let stroke = Stroke::new(z.max(1.0), look.fg);
    painter.rect_stroke(plot, 0.0, stroke, egui::StrokeKind::Middle);
    for t in &t_ticks {
        let x = xs.pos(*t);
        painter.line_segment(
            [
                Pos2::new(x, plot.bottom()),
                Pos2::new(x, plot.bottom() + tick),
            ],
            stroke,
        );
        painter.text(
            Pos2::new(x, plot.bottom() + tick),
            Align2::CENTER_TOP,
            time_label(*t, t_step),
            look.font.clone(),
            look.fg,
        );
    }
    for (v, label) in &y_labels {
        let y = ys.pos(*v);
        painter.line_segment(
            [Pos2::new(plot.left() - tick, y), Pos2::new(plot.left(), y)],
            stroke,
        );
        painter.text(
            Pos2::new(plot.left() - tick - pad / 2.0, y),
            Align2::RIGHT_CENTER,
            label,
            look.font.clone(),
            look.fg,
        );
    }
    if !look.title.is_empty() {
        let h = title_w - pad;
        vertical_text(
            painter,
            Pos2::new(rect.left() + pad + h / 2.0, plot.center().y),
            &look.title,
            look.title_font.clone(),
            look.fg,
        );
    }
}

/// Steps for the time axis, in seconds: whole seconds, minutes and hours.
const TIME_STEPS: &[f64] = &[
    1.0, 2.0, 5.0, 10.0, 15.0, 30.0, 60.0, 120.0, 300.0, 600.0, 900.0, 1800.0, 3600.0, 7200.0,
    10_800.0, 21_600.0, 43_200.0, 86_400.0,
];

/// Ticks from `from` to `to` (seconds since 1970) at the smallest step
/// giving at most `max_ticks`, on whole steps of local time (`offset`
/// seconds ahead of UTC). Returns the ticks and the step.
fn time_ticks(from: f64, to: f64, max_ticks: usize, offset: f64) -> (Vec<f64>, f64) {
    let span = to - from;
    if span <= 0.0 || !span.is_finite() {
        return (Vec::new(), 0.0);
    }
    let step = TIME_STEPS
        .iter()
        .copied()
        .find(|s| span / s <= max_ticks.max(1) as f64)
        .unwrap_or_else(|| 86_400.0 * (span / 86_400.0 / max_ticks.max(1) as f64).ceil());
    let first = ((from + offset) / step).ceil() * step - offset;
    let ticks = (0..)
        .map(|i| first + i as f64 * step)
        .take_while(|t| *t <= to)
        .take(1000)
        .collect();
    (ticks, step)
}

/// How far local time is ahead of UTC at `t`, in seconds.
fn local_offset(t: f64) -> f64 {
    Local
        .timestamp_opt(t as i64, 0)
        .single()
        .map_or(0.0, |d| d.offset().local_minus_utc() as f64)
}

/// A time axis label in local time, as precise as the step.
fn time_label(t: f64, step: f64) -> String {
    let Some(d) = Local.timestamp_opt(t.round() as i64, 0).single() else {
        return String::new();
    };
    let format = match step {
        s if s < 60.0 => "%H:%M:%S",
        s if s < 86_400.0 => "%H:%M",
        _ => "%d %b",
    };
    d.format(format).to_string()
}

// --- the strip chart widget --------------------------------------------------

/// A strip chart's `time_range`, such as `1 minute` or `1 hour 30 min`, in
/// seconds.
pub fn parse_span(text: &str) -> Option<f64> {
    let mut total = 0.0;
    let mut words = text.split_whitespace().peekable();
    let mut any = false;
    while let Some(word) = words.next() {
        // A number and its unit, apart (`10 min`) or together (`10min`).
        let split = word
            .find(|c: char| !(c.is_ascii_digit() || c == '.'))
            .unwrap_or(word.len());
        let (num, unit) = word.split_at(split);
        let n: f64 = num.parse().ok()?;
        let unit = if unit.is_empty() {
            words.next().unwrap_or("s")
        } else {
            unit
        };
        let secs = match unit.to_ascii_lowercase().trim_end_matches('s') {
            "" | "sec" | "second" => 1.0,
            "m" | "min" | "minute" => 60.0,
            "h" | "hour" => 3600.0,
            "d" | "day" => 86_400.0,
            "w" | "week" => 7.0 * 86_400.0,
            _ => return None,
        };
        total += n * secs;
        any = true;
    }
    (any && total > 0.0).then_some(total)
}

/// Draw a `.bob` strip chart: its traces' PVs over the last `time_range`,
/// on its first value axis.
pub fn draw_stripchart(painter: &egui::Painter, node: &Node, rect: Rect, z: f32) {
    let w = &node.w;
    let back = w
        .color("background_color")
        .map(|c| color(&c))
        .unwrap_or(Color32::WHITE);
    fill(painter, rect, back);
    let fg = w
        .color("foreground_color")
        .map(|c| color(&c))
        .unwrap_or(Color32::BLACK);
    let pad = 4.0 * z;
    let top = plot::title(painter, node, rect, fg, z);

    let traces = plot::traces(node, z);
    let shown: Vec<(&plot::Trace, &Series)> = traces
        .iter()
        .zip(&node.series)
        .filter(|(t, _)| t.visible)
        .filter_map(|(t, s)| Some((t, s.as_ref()?)))
        .collect();
    let legend_font = style::font_id(&w.font("legend_font").unwrap_or_default(), z);
    let legend_h = if w.bool("show_legend").unwrap_or(true) && !shown.is_empty() {
        text_size(painter, "Xg", &legend_font).y + pad
    } else {
        0.0
    };
    if legend_h > 0.0 {
        let named: Vec<&plot::Trace> = shown.iter().map(|(t, _)| *t).collect();
        plot::legend(painter, &named, rect, legend_h, pad, &legend_font, fg, z);
    }

    let axis = plot::axis_cfg(
        w.get("y_axes").and_then(|e| e.child("y_axis")),
        "Value",
        node,
        z,
    );
    let span = w
        .text("time_range")
        .map(|t| node.expand(t))
        .and_then(|t| parse_span(&t))
        .unwrap_or(60.0);
    let look = Look {
        log: axis.log,
        range: (!axis.autoscale).then_some(axis.range),
        title: if axis.visible {
            axis.title.clone()
        } else {
            String::new()
        },
        grid: axis.grid,
        fg: axis.color,
        back,
        font: axis.scale_font,
        title_font: axis.title_font,
    };
    let lines: Vec<Line> = shown
        .iter()
        .map(|(t, s)| Line {
            series: s,
            color: t.color,
            width: t.width,
        })
        .collect();
    let area = Rect::from_min_max(
        Pos2::new(rect.left(), top),
        Pos2::new(rect.right(), rect.bottom() - legend_h),
    );
    draw_plot(painter, area, &lines, span, now(), &look, z);
    // Keep time moving while the PVs are quiet, as `Trend::ui` does.
    painter
        .ctx()
        .request_repaint_after(scroll_repaint(span, area.width()));

    if let Some(s) = plot::worst_state(node) {
        alarm_border(painter, w, &s, rect, z);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn samples_keep_gaps_once_and_stay_in_order() {
        let mut s = Samples::default();
        s.push(1.0, f64::NAN);
        assert_eq!(s.len(), 0, "a trend starts with a value");
        s.push(1.0, 5.0);
        s.push(2.0, f64::NAN);
        s.push(3.0, f64::NAN);
        s.push(1.5, 6.0);
        let pts: Vec<(f64, f64)> = s.since(0.0).collect();
        assert_eq!(pts.len(), 3);
        assert!(pts[1].1.is_nan());
        // A clock stepping back does not reorder them.
        assert_eq!(pts[2], (2.0, 6.0));
    }

    #[test]
    fn samples_are_bounded_by_count_and_age() {
        let mut s = Samples::default();
        for i in 0..MAX_SAMPLES + 10 {
            s.push(i as f64 * 0.001, i as f64);
        }
        assert_eq!(s.len(), MAX_SAMPLES);
        assert_eq!(s.since(0.0).next().map(|p| p.1), Some(10.0));
        s.push(MAX_AGE + 200.0, 1.0);
        assert_eq!(s.len(), 1);
    }

    #[test]
    fn since_includes_the_sample_that_holds_at_the_start() {
        let mut s = Samples::default();
        for t in [10.0, 20.0, 30.0, 40.0] {
            s.push(t, t);
        }
        let ts: Vec<f64> = s.since(25.0).map(|p| p.0).collect();
        assert_eq!(ts, [20.0, 30.0, 40.0]);
        assert_eq!(s.since(50.0).count(), 1);
        assert_eq!(s.since(0.0).count(), 4);
    }

    #[test]
    fn series_sample_every_update() {
        let pvs = pv::PvManager::new();
        let pv = pvs.subscribe("loc://trend_test(1)");
        let series = Series::new(pv.clone());
        pv.write("2");
        pv.write("3");
        let samples: Vec<f64> = series.samples.lock().since(0.0).map(|p| p.1).collect();
        assert_eq!(samples, [1.0, 2.0, 3.0]);
        // Drawn (in columns of a second, so maybe thinned): from the
        // first to the last, which holds to now.
        let values: Vec<f64> = series
            .points(0.0, now() + 1.0, 1.0)
            .iter()
            .map(|p| p.1)
            .collect();
        assert_eq!(values.first(), Some(&1.0));
        assert_eq!(values[values.len() - 2..], [3.0, 3.0]);
    }

    /// Points as bits, so gaps (NaN) compare equal.
    fn bits(pts: &[(f64, f64)]) -> Vec<(u64, u64)> {
        pts.iter().map(|p| (p.0.to_bits(), p.1.to_bits())).collect()
    }

    fn thinned(s: &Samples, from: f64, dt: f64) -> Vec<(f64, f64)> {
        let mut t = Thinned::default();
        t.update(s, from, dt);
        t.points().collect()
    }

    #[test]
    fn thinning_keeps_each_columns_ends_and_extremes() {
        let mut s = Samples::default();
        // A day of samples at 1 Hz, as a 1-day strip chart of 1000 pixels
        // keeps: 86 seconds a pixel.
        for i in 0..86_400 {
            let t = i as f64;
            s.push(
                t,
                (t * 0.01).sin() * 10.0 + if i == 5000 { 100.0 } else { 0.0 },
            );
        }
        let dt = 86.4;
        let pts = thinned(&s, 0.0, dt);
        assert!(pts.len() <= 4 * 1001, "{} points", pts.len());
        assert!(pts.windows(2).all(|w| w[0].0 <= w[1].0), "in time order");
        // The spike and the first and last samples survive.
        assert!(pts.contains(&(5000.0, s.since(5000.0).nth(1).unwrap().1)));
        assert_eq!(pts.first(), s.since(0.0).next().as_ref());
        assert_eq!(pts.last().map(|p| p.0), Some(86_399.0));
        // Same extremes as all the samples.
        let max = |p: &mut dyn Iterator<Item = (f64, f64)>| p.map(|p| p.1).fold(f64::MIN, f64::max);
        assert_eq!(max(&mut pts.iter().copied()), max(&mut s.since(0.0)));
    }

    #[test]
    fn thinning_takes_in_only_new_samples_and_keeps_gaps() {
        let mut s = Samples::default();
        let mut t = Thinned::default();
        let push = |s: &mut Samples, t0: f64, n: usize| {
            for i in 0..n {
                let x = t0 + i as f64 * 0.05;
                s.push(x, (x * 3.0).cos());
            }
        };
        push(&mut s, 0.0, 500);
        t.update(&s, 0.0, 0.5);
        s.push(25.0, f64::NAN);
        push(&mut s, 26.0, 500);
        t.update(&s, 0.0, 0.5);
        assert_eq!(t.seen, s.pushed);
        // Taking in the new samples gives what thinning them all at once does.
        let all: Vec<(f64, f64)> = t.points().collect();
        assert_eq!(bits(&all), bits(&thinned(&s, 0.0, 0.5)));
        assert_eq!(all.iter().filter(|p| p.1.is_nan()).count(), 1, "the gap");
        // Time moving on forgets what has scrolled off, but the point that
        // holds at the start.
        t.update(&s, 40.0, 0.5);
        let later: Vec<(f64, f64)> = t.points().collect();
        assert!(later[0].0 < 40.0 && later[1].0 >= 40.0);
        // A new width rebuilds it.
        t.update(&s, 40.0, 0.25);
        let rebuilt: Vec<(f64, f64)> = t.points().collect();
        assert_eq!(bits(&rebuilt), bits(&thinned(&s, 40.0, 0.25)));
    }

    #[test]
    fn plots_scroll_a_pixel_at_a_time_but_not_too_often() {
        let secs = |d: std::time::Duration| d.as_secs_f64();
        // An hour over 600 pixels scrolls a pixel every 6 s: once a second
        // keeps the clock moving.
        assert_eq!(secs(scroll_repaint(3600.0, 600.0)), 1.0);
        // Half a second a pixel.
        assert_eq!(secs(scroll_repaint(300.0, 600.0)), 0.5);
        // A minute over 600 pixels would redraw the display 10 times a second.
        assert_eq!(secs(scroll_repaint(60.0, 600.0)), MIN_SCROLL_REPAINT);
        assert_eq!(secs(scroll_repaint(60.0, 0.0)), 1.0);
    }

    #[test]
    fn disconnected_pvs_leave_a_gap() {
        let off = PvState::default();
        assert!(sample(&off).unwrap().is_nan());
        let on = PvState {
            connected: true,
            value: pv::Value::Double(2.0),
            ..Default::default()
        };
        assert_eq!(sample(&on), Some(2.0));
        let text = PvState {
            connected: true,
            value: pv::Value::Str("idle".into()),
            ..Default::default()
        };
        assert_eq!(sample(&text), None);
    }

    #[test]
    fn time_ticks_fall_on_whole_local_steps() {
        let (t, step) = time_ticks(1000.0, 1600.0, 6, 0.0);
        assert_eq!(step, 120.0);
        assert_eq!(t, [1080.0, 1200.0, 1320.0, 1440.0, 1560.0]);
        // An hour ahead of UTC, hours are whole in local time.
        let (t, step) = time_ticks(0.0, 4.0 * 3600.0, 4, 1800.0);
        assert_eq!(step, 3600.0);
        assert_eq!(t[0], 1800.0);
        // Longer than a day per tick: whole days.
        let (_, step) = time_ticks(0.0, 30.0 * 86_400.0, 5, 0.0);
        assert_eq!(step, 6.0 * 86_400.0);
        assert!(time_ticks(5.0, 5.0, 5, 0.0).0.is_empty());
    }

    #[test]
    fn time_ranges_parse() {
        assert_eq!(parse_span("1 minute"), Some(60.0));
        assert_eq!(parse_span("10 min"), Some(600.0));
        assert_eq!(parse_span("1 hour 30 minutes"), Some(5400.0));
        assert_eq!(parse_span("30s"), Some(30.0));
        assert_eq!(parse_span("2 days"), Some(172_800.0));
        assert_eq!(parse_span("soon"), None);
        assert_eq!(parse_span(""), None);
    }
}